Mitch Daniels, as head of the Office of Management and Budget at the White House in 2002, had one of the hardest jobs in Washington – particularly after 9/11. He was constantly turning down requests for money. So when he asked me, "How much do you need?", I knew this was no time for timidity. My team had been tasked with the largest reorganization of the government since the Truman administration. We needed a lot of money. The ask was $20 billion. Mitch didn't bat an eye. "That sounds OK," he told me.

This negotiation ultimately led to the creation of the Department of Homeland Security, which officially opened its doors 15 years ago. We combined 22 agencies tackling everything from layered security at air, land and seaports to protecting critical infrastructure.

Now, our greatest terror threat is no longer from a large coordinated attack such as the one we witnessed on 9/11, but rather from the lone wolf attack  emanating from here at home. Much more common now are mass shootings – in our schools, our workplaces, and where large crowds gather. Communities from Connecticut to Nevada to, just recently, Parkland, Fla., have been devastated by madmen who lived among them. These "homegrown" attacks illustrate just how dramatically the threat landscape has changed since those days when we were building a new department focused on preventing more jet airplanes from becoming weapons.

DHS must, of course, continue to focus on its core mission – protecting the homeland from terrorism.  At the same time, the federal government also must play a role in finding solutions to these mass shootings, like the one at a Florida high school that killed 17 children and adults.

I supported an assault weapons ban as a member of Congress in the 1990s and still believe gun control should be a given in any discussion on reducing these mass shootings. As a Vietnam veteran trained to be an expert marksman on an M-16, I can assure you assault weapons are killing machines, not hunting weapons. For me, however, it's a little more complicated than just assault weapons bans and background checks.

People understandably talk about the Second Amendment, but we are long overdue for a national discussion about privacy and whether individuals have a right to keep their own medical history to themselves, or under certain circumstances are obliged to provide it. We have reached a tipping point on mental health and privacy. The debate has to be far deeper than assault weapons.

I had hoped that discussion would have begun back in 2007 when I served on a panel that investigated the mass shooting at Virginia Tech that left 32 students dead. Today's rhetoric sounds awfully familiar to what we heard back then. We need to get serious about looking for ways as a society to respect the right to bear arms, but to ensure those who have them are properly trained, and that access is appropriately limited for those who have mental health problems. One thing is for certain: arming teachers is not the solution.

As we built the Department of Homeland Security in 2002 and 2003, we knew then what is still true today – perhaps even more so. We cannot secure the homeland from inside the Beltway. We need to work together to share information – both between the public and private sectors and between federal and state agencies – to prevent attacks when we can and to quickly respond when bad things do happen.

Fifteen years after Homeland Security opened its doors, we now better understand that terrorism is a global scourge. It is a reality we will confront for the foreseeable future. The one constant amid these changes is America's resiliency. We don't live in fear. We live in freedom. As long as we remain resilient, those who seek to do harm can never take our freedoms away.

Tom Ridge, a former Pennsylvania governor, became the first assistant to the president for homeland security and, later, the first secretary of the U.S. Department of Homeland Security after that department was established in 2003 following the events of 9/11.   Ridge is founder and chairman of Ridge Global in Washington.